Low power gas detector

ABSTRACT

A gas detector includes a gas detecting element comprised of a metal oxide semiconductor material which changes its value of electrical resistance at an elevated temperature when it absorbs a gas. In one form, the gas detecting element is supported in the form of a bridge so as to increase its response speed by making thermal capacity as small as possible. In another form, the gas detecting element is supported in the form a cantilever, more preferably in the form of a ring as formed along the periphery of a overhang portion of a disc-shaped layer of electrically insulating material. Also provided is a process for manufacturing a thin film of metal oxide, which may be advantageously used as a gas detecting element of a gas detector or a transparent electrode, for example, in a liquid crystal panel.

This is a divisional application based on copending application Ser. No.577,858 filed Feb. 7, 1984 now U.S. Pat. No. 4,580,439.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a gas detector for detecting the presence of aparticular gas such as a combustible gas and a hazardous gas beyond acertain level and a method for manufacturing the same. In particular,the present invention relates to a low power gas detector of thesemiconductor type suited for use as a gas leak alarm for detecting thepresence of excessive amount of gas such as LP gas and commercial orutility gas and giving a warning signal upon detection. Morespecifically, the present invention relates to a method formanufacturing a metal oxide thin film which may be advantageously usedas a gas detecting element in a semiconductor type gas detector or atransparent electrode film in a display panel or photoelectric sensor.

2. Description of the Prior Art

A gas detector using a metal oxide semiconductor such as SnO₂ and ZnO iswell known. In such a prior art gas detector, electrodes and/orcoil-shaped electrodes also serving as heater coils are provided asburied in the body of metal oxide semiconductor, wherein changes in theresistance of the metal oxide semiconductor due to absorption of aparticular gas at the surface are used to detect the presence oroveramount of a particular gas. However, one of the particulardisadvantages in the prior art gas detector has been the large powerrequirement. For example, none of the prior art gas detectors has beensuited for use with batteries. Thus, there has been a need fordeveloping a low power gas detector which may be driven by batteries foran extended period of time.

As a gas detecting element of a semiconductor type gas detector, use hasbeen commonly made of a sintered metal oxide semiconductor. As describedin the Japanese Patent Laid-open Publication No. 58-30648, the typicalmethod for manufacturing such a gas detecting element is to produce tinoxide by processing tin with dense nitric acid and then a sediment oftin oxide thus obtained is sintered using a binder such as SiO₂ and Al₂O₃. However, as described above, instead of the prior art gas detectordriven by a commercial line voltage, research has been and still isbeing carried out to develop a battery-driven gas detector. Under thecircumstances, it is required to develop a gas detector smaller in scaleand thus lower in power consumption. In such a miniaturized gasdetector, a gas detecting element as thin as a few microns and as smallin area as some hundreds of microns squared must be fabricated. None ofthe prior art techniques is capable of fabricating such a small-sizedgas detecting element.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide animproved gas detector and a method for manufacturing the same.

Another object of the present invention is to provide an improvedsemiconductor type gas detector.

A further object of the present invention is to provide a battery-drivengas detector which is low in power consumption and small in size.

A still further object of the present invention is to provide ahigh-sensitivity gas detector excellent and stable in operation and longin service life.

A still further object of the present invention is to provide a gasdetector which is suited for mass production and thus remarkably low inunit cost.

A still further object of the present invention is to provide a methodof forming a desired pattern of metal oxide semiconductor film which isparticularly suited for use as a gas detecting element or a transparentelectrode film.

A still further object of the present invention is to provide a methodof forming a film of metal oxide semiconductor having an extremely finepattern.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a gas detector constructed in accordancewith one embodiment of the present invention;

FIG. 2 is a cross-sectional view taken along line I--I shown in FIG. 1;

FIG. 3 is a schematic illustration showing the overall structure of thegas detector of FIG. 1 which is useful for explaining its operation;

FIG. 4 is a schematic illustration showing the electrical circuitstructure of the gas detector of FIG. 1;

FIGS. 5a and 5b are graphs showing experimental data obtained in thecircuit of FIG. 4;

FIG. 6 is an enlarged schematic illustration showing the central portionof the structure shown in FIG. 3;

FIGS. 7 through 14 show steps of a process for manufacturing the gasdetector 10 of FIG. 1 in accordance with one embodiment of the presentinvention wherein FIGS. 7, 9, 10, 12, 13 and 14 are cross-sectionalviews of the structure at each step and FIGS. 8 and 11 are plan viewsshowing photo-mask patterns used during the process;

FIG. 15 is a perspective view showing another gas detector constructedin accordance with another embodiment of the present invention;

FIGS. 16 and 17 are cross-sectional views taken along lines IV--IV andIV'--IV', respectively, shown in FIG. 15;

FIGS. 18 and through 21 are cross-sectional views showing the structureat each step in a process for manufacturing the gas detector 20 of FIG.15;

FIGS. 22 and 23 are schematic plan views showing photo-mask patternsused during the process for manufacturing the gas detector 20 of FIG.15;

FIG. 24 is a schematic plan view showing a modification of the structureshown in FIG. 15;

FIG. 25 is a cross-sectional view taken along line V--V shown in FIG.24;

FIG. 26 is a schematic plan view showing a gas detector constructed inaccordance with a further embodiment of the present invention;

FIG. 27 is a cross-sectional view taken along line VI--VI shown in FIG.26;

FIGS. 28a through 28h are cross-sectional views showing steps of forminga gas detecting semiconductor film on a heat-resistant substrate inaccordance with one method of the present invention;

FIGS. 29a through 29h are cross-sectional views showing steps of forminga gas detecting semiconductor film on a bridge structure in accordancewith another method of the present invention;

FIGS. 30a through 30g are cross-sectional views showing steps of forminga transparent electrode film in a liquid crystal display panel inaccordance with a further method of the present invention;

FIG. 31 is a plan view showing a gas detector constructed in accordancewith a still further embodiment of the present invention;

FIG. 32 is a cross-sectional view taken along line I'--I' shown in FIG.31;

FIG. 33 is a schematic illustration showing part of the structure shownin FIG. 31;

FIG. 34 is an electrical circuit structure of the gas detector shown inFIG. 31;

FIG. 35 is a cross-sectional view taken along line II'--II' shown inFIG. 31;

FIG. 36 is a schematic illustration showing part of heater sections onan enlarged scale useful for explaining the manner of heat dissipation;

FIG. 37 is a schematic plan view showing a gas detector constructed inaccordance with a still further embodiment of the present invention;

FIG. 38 is a cross-sectional view taken along line III'--III' shown inFIG. 37;

FIG. 39 is a schematic illustration which is useful for explaining theprinciple of operation of some embodiments of the present invention;

FIG. 40 is a graph showing the characteristic obtained along the currentpath indicated by the solid line in FIG. 39;

FIG. 41 is a circuit diagram showing the driving circuit which may beused with the present gas detector;

FIGS. 42 through 48 are cross-sectional views showing steps of oneprocess of manufacturing the gas detector of FIG. 37 in accordance witha still further embodiment of the present invention;

FIG. 49 is a schematic plan view showing a gas detector constructed inaccordance with a still further embodiment of the present invention;

FIG. 50 is a cross-sectional view taken along line IV'--IV' shown inFIG. 49;

FIG. 51 is a schematic plan view showing a gas detector constructed inaccordance with a still further embodiment of the present invention;

FIGS. 52 through 56 are cross-sectional views showing steps of a processof manufacturing a still further embodiment of the present gas detector;

FIGS. 57 through 59 are cross-sectional views showing steps of a processof manufacturing a still further embodiment of the present gas detector;

FIG. 60 is a plan view of the gas detector shown in FIG. 59;

FIG. 61 is a cross-sectional view showing a still further embodiment ofthe present gas detector;

FIGS. 62 through 66 are cross-sectional views showing steps of a processof manufacturing a still further embodiment of the present gas detector;

FIG. 67 is a plan view of the gas detector shown in FIG. 66; and

FIGS. 68 through 70 are cross-sectional views showing steps of a processof manufacturing a still further embodiment of the present gas detector.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown the overall structure of a gasdetector 10 constructed in accordance with one embodiment of the presentinvention as mounted in a supporting structure. FIG. 2 is across-sectional view taken along line I--I indicated in FIG. 1. The gasdetector 10 of the present invention generally comprises a substrate 1,an insulating layer 2 and a metal layer 3. The metal layer 3, in fact,is patterned into three strips 7, 8 and 9 arranged side by side, eachincluding an elongated section and a pair of electrode or pad sectionsprovided on both ends of the elongated section. Among these threestrips, the elongated sections of the strips 7 and 9 serve as heaterswith the sections 7a, 7d, 9c and 9f serving as pads, and the elongatedsection of the strip 8 serves as a gas detecting element with thesections 8b and 8e serving as pads thereof. The gas detector 10 alsoincludes a gas detecting semiconductor layer 6 and an insulating layer4. In registry in location with the electrode sections 7a, 8b, 9c, 7d,8e and 9f, there are provided holes in the insulating layer 4 and bumps5 are formed in the holes.

As best shown in FIG. 2, the gas detector 10 is mounted on a supportingstructure, or film carrier 20 in the illustrated embodiment, with thebumps 5 of the electrode sections 7a, 8b, 9c, 7d, 8e and 9f bonded bythermocompression to respective leads 26 projecting from the filmcarrier 20. The gas detector 10 is covered by an anti-explosion net 24and a dust-off filter 25 at its top and by a bottom cover 21 at itsbottom. Thus, the gas detector 10 is provided as effectively enclosed.The anti-explosion net 24 and the dust-off filter 25 are fixedlyattached to a top cover frame 22 which in turn is fixedly mounted on thefilm carrier 20 and they allow passage of gas therethrough. The dust-offfilter 25 is provided so as to prevent the gas detector 10 frommalfunctioning due to deposition of debris and foreign matter on thesurface because the gas detector 10 has a fine structure. In thepreferred embodiment, glass wool is used as the dust-free filter 25 sothat debris of 0.1 microns or more are prevented from passingtherethrough but it does not present any problem for gas to passtherethrough. As is obvious, gas may be introduced into the space asdefined by the supporting structure by freely passing through both ofthe anti-explosion net 24 and dust-free filter 25 to be partly absorbedby the gas detecting semiconductor element 6.

With reference to FIGS. 3 and 4, the operation of the gas detector 10will be described. As mentioned before, the gas detector 10 has threestrips extending substantially linearly and arranged side by side. Theside strips 7 and 9 serving as heaters are provided with electrodesections 7a, 7d and 9c, 9f, respectively, on both ends, and the centerstrip 8 is a gas detecting lead which is also provided with a pair ofelectrode sections 8b and 8e on both ends. As shown in FIG. 4, a voltagesupply and pulse driving circuit 11 is connected as shown. Such asandwiching arrangement in which the gas detecting lead 8 is provided assandwiched between the side heater strips 7 and 9 one on each side ispreferable because temperature may be maintained uniform across the gasdetecting semiconductor layer 6. It should be noted, however, that thepresent invention should not be limited only to such arrangement. Aslong as uniform distribution of temperature is maintained, various otherarrangements may be employed; for example, heater and gas detectingstrips may be alternately provided as many as desired.

With a voltage pulse of 1.5-3V applied between the electrode sections 9cand 9f (also between the electrode sections 7a and 7d), current Icfflows, so that the heaters 7 and 9 quickly reaches the temperatureranging from 350° C. to 400° C. in 1-4 milliseconds. As a result, theheat thus produced is transmitted through the insulating film 2 to thegas detecting semiconductor layer 6 thereby causing the layer 6 to bealso heated, so that the electrical resistance of layer 6 becomesdecreased. When the gas detecting semiconductor layer 6 absorbs aparticular kind of gas, its resistance becomes lowered in the order ofmagnitude by two to three, so that part of the driving current Icfflowing through the heater strip 7 (and 9) flows into the gas detectingstrip 8 thereby creating a flow of current Icbef, which may be detectedas a change in voltage in the form of a pulse between the electrodes 8band 8e. Accordingly, the concentration of gas may be detected byobserving a change in voltage between the electrodes 8b and 8e.

FIGS. 5a and 5b are graphs showing the wave form of an input voltagepulse V_(IN) applied between the electrodes 9c and 9f and the wave formsof resulting output voltage pulses V_(OUT0) for 0% in gas concentrationand V_(OUT1) for 0.35% in gas concentration. When a gas of highconcentration, e.g., a gas of 100% in concentration is absorbed, Icbefincreases and Icf decreases. With a reduction of Idf, there occurs adecrease in temperature in the heater strips 7 and 9, which then causesthe temperature of gas detecting semiconductor layer 6 to lower and thusits resistance to increase thereby making Icbef smaller again. Such abelated reduction is preferable because it contributes to shorten thetime required for the output voltage at the electrodes 8b and 8e tostabilize and to limit the temperature increase of heater strips 7 and9. Such an advantage stems partly from the fact that the heater strips 7and 9 are very small in thermal capacity and thus only a short timeperiod is required for them to reach the thermal equilibrium state andpartly from the particular driving scheme employed.

FIG. 6 is an enlarged view of the pattern of three strips 7-9 shown inFIG. 3. The illustrated strips 7-9 may be preferably dimensioned with 1ranging 100 to 500 microns, m ranging from 1 to 3 microns, n rangingfrom 5 to 20 microns, s ranging from 10 to 50 microns, t ranging from 15to 50 microns and u ranging from 1 to 3 microns. As may be noticed, thegas detecting strip 8 is narrower with its width m set in the rangebetween 1 and 3 microns. Such a structure is desired so as to improvethe S/N ratio of a detecting voltage because the output voltage betweenthe electrodes 8b and 8e may be increased by increasing the resistanceof gas detecting center strip 8 and to improve the uniformity oftemperature distribution therealong. The electrical resistance of strip8 between 8b and 8e becomes larger as the width of the strip 8 becomesnarrower. Thus, a larger detected voltage signal is obtained for a givendetection current as compared with the case in which the strip 8 iswider. It is to be noted that the side heater strips 7 and 9 areprovided with widened portions approximately at the midway between theend electrodes. This is so structured as to lower the current density atthe central portion where the temperaure tends to be higher than therest by increasing the cross-sectional area of the strip 8. With suchwidened portions provided at the center of the heater strips 7 and 9,the heat produced at the central portions of the heater strips 7 and 9may be reduced by a controlled amount, and as a result the distributionof temperature may be made remarkably uniform along their longitudinaldirections. Such a uniform temperature distribution also contributes toprevent the so-called electromigration from occurring, which thencontributes to secure an extended service life of heater strips 7 and 9.It should further be noted that the widened portions provided at thecenter of the heater strips are actually formed by projectionsprojecting inwardly toward the gas detecting strip 8 in the illustratedembodiment. With such a structure, the distance between the side heaterstrip 7 (and 9) and the center detecting strip 8 is minimized so thatthe resistance of the gas detecting semiconductor layer 6 presentedbetween the side and center strips 7 (9) and 8 may also be minimizedthereby allowing to obtain an increased detecting output voltage betweenthe electrodes 8b and 8e which contributes to enhance its S/N ratio.Such a structure is particularly advantageous because the gas detector10 may be made less sensitive to humidity and alcohol which areexpecially reactive at low temperatures when the distance between thestrips 7 and 8 becomes smaller, the overall width t becomes smaller andthus the thermal capacity of the gas detecting semiconductor layer 6becomes smaller. As a result, the layer 6 can be heated rapidly and thusthe time required for the layer 6 to reach a condition for initiation ofdetection is short.

Accordingly, the time in which noise is produced can be minimized.

Now, a process for manufacturing the gas detector 10 having theabove-described structure in accordance with one embodiment of thepresent invention will be described with reference to FIGS. 7 through14. In the first place, as shown in FIG. 7, on a substrate 1 is formedan insulating layer 2, a metal layer 3 and a resist layer 4 in the ordermentioned one on top of another. The substrate 1 constitutes a basestructure of the gas detector 10 and its supports the strip pattern andthe associated electrode pads, and it preferably comprises a materialwhich may be easily subjected to undercut etching without significantlyaffecting the overlying structure and which does not alter in propertyas well as in shape at high temperatures, e.g., by heating to 500° C.for a time period of a few to 10 hours. In the present embodiment, useis made of Si (100), but any other material such as Al, Cu, N and Cr mayalso be used. The substrate 1 illustrated is square in shape, having oneside measuring 1-4 mm and a thickness of 0.1-1 mm. It is true thoughthat the substrate 1 is thinner the better because it may be easilysplit when produced in mass.

The insulating layer 2 is provided to support the strip pattern thereonas electrically isolated from the substrate 1 and to provide electricalinsulation between electrically conductive strips. The insulating layer2 preferably comprises a material which is highly electrically resistiveas well as heat-resistant and is similar in thermal expansioncoefficient to the heater strip material. For example, such aninsulating material may be selected from the group consisting of Al₂ O₃,MgO, Si₃ N₄ and Ta₂ O₅. In the present embodiment, the insulating film 2is formed for SiO₂ by RF sputtering (Ar pressure 0.1-0.01 Torr, inputpower density 1-10 W/cm², substrate temperature 350°-400° C.) to thethickness of 0.3-2 microns.

The metal layer 3 in fact has a three-layer structure includingunderlying and overlying layers 3a, 3a and an intermediate layer 3bsandwiched between these layers 3a, 3a. The underlying layer 3a isprovided to increase adherence between the intermediate layer 3b and theinsulating layer 2 and it comprises a material which is resistant toboth of etchants used to etch the substrate 1 and the insulating layer2. In the present embodiment, the underlying metal layer 3a is formedfrom Mo by RF sputtering under the same conditions as described above tothe thickness of 300-800 angstroms. Alternatively, other materials suchas Cr, Ni and Ti may be used for forming the layer 3a. The intermediatelayer 3b will be formed into a heater strip so that it preferablycomprises a material which may remain stable in property for an extendedperiod of time. In the present embodiment, the intemediate layer 3b isformed from Pt by RF sputtering under the above-mentioned conditions tothe thickness of 0.3-2 microns. As alternatives, such materials as SiCand TaN₂ may be used. After formation of the intermediate layer 3b, theoverlying contact layer 3a is formed thereon under the same conditionsas mentioned above.

The resist layer 4 is formed on top of the three-layer structured metallayer 3 and it serves not only as a mask at the time of dry-etching themetal layer 3 but also as a solder bump glass dam at the time of formingan insulation for the gas detecting strip and a bump in each electrodepad. In the present embodiment, the resist layer 4 is formed from SiO₂by RF sputtering under the above-mentioned conditions to the thicknessof 0.5-1 microns.

It is to be noted that the above-described steps may be carried outcontinuously in the same batch and thus suitable for application to amass production scheme. When processed continuously, the interfacebetween the two adjacent layers may be maintained clean and thusprovides excellent contactability. Moreover, by holding the substratetemperature in the range between 350° and 400° C. during RF sputtering,resulting films may be made more densely thereby allowing to prevent theresistance of metal layer 3 from fluctuating due to aging, and,furthermore, thermal stress produced in the film during operation may beminimized because the operating temperature of gas detector 10 is in therange between 350° and 400° C. Accordingly, reliability in operation issignificantly enhanced.

Then the layer 4 of SiO₂ is etched by well known photolithographictechnology, using common buffered fluoric acid (HF+NH₄ F) as an etchingsolution. The photomask to be used includes a pair of parallely arrangedheater strip patterns and a gas detecting strip pattern located assandwiched between the pair of heater strip patterns, each having anelongated section and a pair of electrode sections on both ends of theelongated section. The patterns are determined such that the gasdetecting strip may be uniformly heated by the sandwiching side heaterstrips and that the substrate 1 in the vicinity of the heater stripsexcepting the electrode pad sections may be undercut when the substrate1 is subjected to anisotropic etching. FIG. 8 shows an example of such aphotomask 12 having desired patterns. Since Si (100) is used as thesubstrate 1, if the electrode sections 12' are defined on a (111) planewhich is difficult to be etched, under a to-be-formed bridge section 12"which is inclined at 45° with respect to the electrode sections 12'located on a (110) plane which is easily etchable. Thus, this (110)plane is undercut when etched thereby forming a void under theto-be-formed bridge section 12". In the case where Si (111) is used asthe substrate 1, if the angle formed between the electrode sections 12'and the bridge section 12" in the photomask 12 is set at 15°, then thedesired undercutting may be effected.

FIG. 9 shows in cross-section the structure taken along line II--IIindicated in FIG. 8 after photoetching the SiO₂ film 4. Afterdry-etching the metal layer 3 using the remaining patterned SiO₂ film 4as a mask, the resulting structure is shown in FIG. 10. The use ofdry-etching is preferred in this step because Pt is difficult to bewet-etched. Ar sputter etching may be preferably used (Ar pressure0.1-0.01 Torr, input power density 1-10 W/cm² and substrate at roomtemperature); however, any other method such as plasma etching with CF₄+O₂ may also be used. Then using a photomask 13 shown in FIG. 11, theSiO₂ layers 2 and 4 are selectively etched to define electrode padsusing openings 7a", 8b", 9c", 7d", 8e" and 9f" and the substrate 1 isselectively etched using openings 14 and 15. The photomask 13 is alsoprovided with openings 16 for defining a pattern of heater and detectorstrips. After such photoetching is carried out, the resulting structuretaken along line III--III is shown in FIG. 12. As shown, the metal layer3 in fact has a three layer structure and is comprised of Pt layer 3asandwiched by a pair of Mo layers at its top and bottom.

Then using the SiO₂ layers 2 and 4 as masks, the Si substrate 1 issubjected to anisotropic etching. As an anisotropic etchant, use may bemade of KOH, NaOH (30-60% aqueous solution, liquid temperature 80°-150°C.), APW (ethylenediamine+pyrocatechol+water, liquid temperature90°-110° C.), hydrazine aqueous solution (64 mol %, liquid temperature90°-100° C.), etc. As shown in FIG. 13, after etching for 20 to 40minutes, the substrate 1 below the SiO₂ film 2' becomes undercut to thedepth of 50 to 300 microns to provide void space thereby forming aprofile of bridge structure by the heater and detector strips 7-9. sucha bridge structure is defined due to a particular relation between thecrystal orientation of the substrate 1 and the pattern of photomask 13.It is to be noted that the openings 14 provided in the photomask 13 ofFIG. 11 contribute to provide a sharp edge profile when the substrate isso etched. For example, in the present embodiment, Si (111) tends toremain on both ends of the bridge section during etching; however, theprovision of openings 14 help etch these portions effectively so thatthe etching time may be reduced to half, which in turn contributes tomitigate the damages which might be imparted to the other portions ofthe structure by the etchant.

If the etched profile is sharp particularly at the ends of the bridgesection, there will be less heat conduction from the heater layer 3 tothe underlying substrate 1 through the insulating layer 2 so thatheating efficiency by the heater strips 7 and 9 may be increased, whichthen helps to obtain a uniform distribution of temperature especiallyalong the longitudinal direction of the detector strip. The end portionsof the bridge section are relatively lower in temperature as comparedwith the central portion; however, sensitivity to humidity and alcoholmay still be maintained low if the gas detecting semiconductor element 6is formed mostly at the central portion of the bridge section and not onthe end portions. FIG. 13 shows the structure which may be obtainedafter subjecting the structure of FIG. 12 to anisotropic etching therebycausing undercutting preferentially under the bridge section 2'. Then Snor Au is vapor-deposited to fill the pad holes 17a and 17b to thethickness of few to 10 microns and the substrate 1 is heated to400°-600° C. to form dome-shaped bumps 5. In this instance, making useof the fact that the mask or resist layer 4 of SiO₂ defines a pattern ofglass dam and is poor in wetability with metal, bumps 5 of Au-Sneutectic alloy may be formed as dome-shaped as shown in FIG. 14.

In FIG. 14 is shown the semiconductor layer 6 which is provided asfilling the gaps between the center and side strips. The semiconductorgas detecting layer 6 may be formed from a metal oxide material such asSnO₂, Fe₂ O₃ and ZnO by sputtering, evaporation or the like to thethickness of 0.3-3 microns, or, alternatively, it may be formed byhaving fine powder of one of the above materials dispersed in a mixtureof water and alcohol and applying such a dispersion by spin coating. Theprocess described above requires only two kinds of photomasks and twokinds of evaporation masks and yet accuracy in mask alignment is not sosevere and in the order of ±3 microns. As is apparent, the manufacturingprocess of the present invention is much simpler as compared with thewell known IC and LSI processes, so that the present process is low incost and high in reliability.

Now, turning to FIG. 15, another embodiment of the present inventionwill be described in detail. FIGS. 16 and 17 are cross-sectional viewstaken along lines IV--IV and IV'--IV', respectively. Similarly with theprevious embodiment, this gas detector also includes a substrate 21, aninsulating layer 22 and a metal layer 23. The metal layer 23 ispatterned such that it incldues a disc-shaped electrode 29f, aring-shaped heater strip 27 of an electrically conductive material whichis generally concentric with the disc-shaped electrode 29f and which hasone end connected from the disc-shaped electrode 29f and the other endconnected to an electrode pad 29b, a ring-shaped detector strip 28 of anelectrically conductive material which is generally concentric with thedisc-shaped electrode 29f and the ring-shaped heater strip 27 and whichhas one end connected from the disc-shaped electrode 29f and the otherend connected to another electrode pad 29e, and a separate electrode pad29g. It is to be noted that as shown in FIG. 17, the disc-shaped centerelectrode 29f is electrically connected to the separate electrode pad29g through the substrate 1 and via the holes provided in the insulatinglayer 22.

A gas absorbing semiconductor layer 26 is provided to fill the gapbetween and on the ring-shaped heater and detector strips 27 and 28. Thesubstrate 21 is undercut around the periphery of the generallydisc-shaped insulating layer 22 so that that portion of the insulatinglayer 22 on which the ring-shaped heater and detector strips 27 and 28and the semiconductor layer 26 are formed extends into the air, as shownin FIGS. 16 and 17. Of importance, the heater strip 27 is located at aposition which is not in contact with and separated away from thesubstrate 21 also serving as a heat sink as much as possible. As will beunderstool later, the separate electrode pad 29g is commonly used forheating and detection. The principle of gas detecting operation in thestructure of FIG. 15 is substantially identical to that in the case ofFIG. 4. In other words, in FIG. 15, a driving voltage pulse is appliedbetween the electrodes 29b and 29f thereby causing the ring-shapedheater strip 27 to be heated, which, in turn, causes the gas detectingsemiconductor layer 26 to be heated. As described before, when thesemiconductor layer 26 becomes heated, its resistance drops by two tothree orders of magnitude, so that the driving current 27 leaks more tothe detector strip 28 thereby forming a voltage pulse between thedetector electrodes 29b and 29e.

FIGS. 18-21 are cross-sectional views showing the structure at severalsteps in the process of manufacturing the gas detector of FIG. 15 takenalong line IV--IV. As shown in FIG. 18, the insulating layer 22 of SiO₂is formed on the substrate 21 of Si (100) by sputtering. Then using aphotomask 14 of FIG. 22, the insulating layer 22 is selectively removedthereby defining an undercut etching opening 22' and contact holeopenings 22f and 22g. Then, as shown in FIG. 19, the three layerstructure including the sandwiching contact layers 23a of Mo and thesandwiched layer of Pt and the resist or mask layer 24 of SiO₂ areformed one after another by sputtering. Then using a photomask 15 ofFIG. 23 provided with ring-shaped heater strip pattern 27', ring-shapeddetector strip pattern 28', and electrode patterns 29e', 29b', 29f' and29g', photoetching is carried out to have the mask layer 24 patterned.Then using the thus patterned mask layer 24 as a mask, the compositemetal layer 23 having the three layer structure is dry-etched and itsresulting structure is shown in FIG. 20.

Then since the patterned mask layer 24 on the composite metal layer 23is very thin, it is completely removed when dipped into an etchant forSiO₂. Thereafter the Si substrate 21 is subjected to anisotropic etchingto undercut the substrate 21 around the insulating layer 22 so that theperipheral portion of the insulating layer 22 becomes projected into theair whereby the ring-shaped heater and detector strips 27 and 28 becomelocated on that peripheral portion of the insulating layer 23 whoseunderside is not adjacent to the substrate 21. Then the semiconductorstrip 26 is formed along the ring-shaped heater and detector strips 27and 28 to provide the structure shown in FIG. 21. In the presentembodiment, since the electrodes 29g and 29f are to be electricallyconnected through the substrate 21, the substrate 21 preferablycomprises a material having high electrical conductivity. For example,Si highly doped with an impurity such as B and P may be used, or a metalsuch as Al, Cu, Ni, Cr, etc. may also be used.

More specifically, the photomasks 14 and 15 shown in FIGS. 22 and 23,respectively, are preferably sized as the diameter φ₁ of disc-shapedelectrode 29f' to be 30-800 microns with the ring-shaped detector strip28' having the width of 1-10 microns located generally concentricallywith and radially outside of the disc-shaped electrode 29f' and thering-shaped heater strip 27' having the width of 3-50 microns locatedgenerally concentrically with and radially outside of the ring-shapeddetector strip 28'. The gap between the rings 27' and 28' is preferablyin the range between 1 and 10 microns. As mentioned earlier, thedisc-shaped insulating layer 22 serving as a support for the rings 27'and 28' is also generally concentric with the disc-shaped centerelectrode 29f' and its diameter is preferably in the range between 50and 1,000 microns. The ring-shaped strips 27 and 28 are provided wtihrespective lead-out portions which are connected to the electrode pads29e and 29b, respectively. As indicated in FIG. 22, the angle formedbetween each of the lead-out portions and one side of the square-shapedelectrode pad, e.g., 22g, is 45°. This is because, as described withrespect to the previous embodiment, such a particular relation betweenthe crystal orientation of Si (100) and the masking pattern allows tohave that portion of the substrate 21 which is generally located belowthe lead-out portions preferentially undercut when subjected toanisotropic etching. The length q of such a lead-out portion ispreferably set at 5-50 microns.

The present embodiment having a generally circular structure isadvantageous in obtaining a uniform distribution of temperature. Thatis, with the provision of the ring-shaped heater strip 27, since heatproduced by the heater strip 27 is uniformly directed to its center,there will be more uniformity in temperature distribution as comparedwith the case of a linear heater strip. Furthermore, the presentembodiment is superior in mechanical durability than the liner heaterstrip type having a bridge-formed supporting structure. This is evenmore true in the case where the longer heater and detector strips aredesired. For example, for the heater strip having the width ranging from3 to 10 microns and the thickness of 0.3 microns with the valve ofresistance at 200 ohms, it must be at least 0.5 mm long. In the case ofa straight heater strip, the longer, the higher the influence of thermalexpansion. In particular, in the case where the heater strip is drivenby pulses, the heater strip will be set in vibration in association withthe frequency of application of driving pulses. Such a vibration isdisadvantageous because the heater strip may be separated away from thesemiconductor layer or cracks may be formed in the insulating layer onwhich the heater strip is supported. On the other hand, the circularlyshaped or coil-shaped heater strip as discussed above does not sufferfrom these disadvantages since it can absorb thermal expansion.

FIG. 24 shows another embodiment of the present invention which has aring-shaped heater strip and thus is similar to the embodiment shown inFIG. 15. FIG. 25 is a cross-sectional view taken along line V--Vindicated in FIG. 24. As shown, a gas detector 30 in this case includesa substrate 31 which is provided with a circular recess 33 at itscenter. Such a circularly shaped recess 31 may be provided by subjectingthe substrate to an anisotropic etching. On the substrate 31 is formedan insulating layer 32 which is also provided with a circular openingconcentrically with the circular recess 33. However, as best shown inFIG. 25, the opening of the insulating layer 32 is smaller in diameterthan the circular recess 33 so that the inner peripheral portion of thecircular opening defines a projection which extends into the air. Onsuch a projection is formed ring-shaped heater and detector strips 37and 38 as spaced apart from each other at a predetermined clearance inthe radial direction. A gas detecting semiconductor layer 36 is formedalong and on the strips 37 and 38. Also provided are a pair of detectorelectrode pads 39b and 39e connected on both ends of the detector ring38 and a pair of heater electrode pads 39c and 39f connected on bothends of the heater ring 37.

FIG. 26 shows a further embodiment of the present invention and FIG. 27is a cross-sectional view taken along line IV--IV indicated in FIG. 26.In these figures, 41 is a substrate; 42, 42' insulating layers; 43 ametal layer; 46 a gas detecting semiconductor layer; 47 ring-shapedheater strips; 48 ring-shaped detector strips; 49a, 49e, 49c and 49gheater electrode pads; 49b, 49f, 49d and 49h detector electrode pads. Infabrication of this device, on both sides of the substrate 41 are firstformed insulating layers 42 and 42', and the bottom insulating layer 42is patterned to form a center opening through which the substrate 41 isetched until the top insulating layer 42' is reached. Then the metallayer 43 having the previously described three-layer structure is formedon the top insulating layer 42' which is then patterned to definering-shaped heater and detector strips 47 and 48 generally along theinner periphery of the circular recess formed in and through thesubstrate 41. The electrode pads 49a-49h are defined at the same time.Then the gas detecting semiconductor layer 46 is formed along thering-shaped strips 47 and 48. In this case, a plurality of the heaterand detector rings 47 and 48 (two for each in the illustrated example)are alternately provided concentrically. However, three or more of suchrings may also be provided if desired. This embodiment is particularlyadvantageous because its mechanical durability is very high against thestresses imparted to the insulating layer 42' due to thermal expansionof the heater rings 47 and externally applied vibrations.

Now, in accordance with another aspect of the present invention, variousprocesses for forming a metal oxide semiconductor film which may be usedto define a fine pattern will be described.

FIGS. 28a through 28h are cross-sectional views each showing thestructure at each step during a process for manufacturing a gasdetecting semiconductor film on a heat-resistant substrate in accordancewith the present invention. FIG. 28a shows a starting structure and itcomprises a substrate 51a of a heat-resistant material such as ceramicsand glass and a metal film 51b formed on the substrate 51a from a metalsuch as Ta₂ N, SiC, NiCr and Pt by thin film forming technology wellknown to one skilled in the art. Although not shown specifically, itshould be understood that the metal layer 51b has been appropriatelypatterned and thus heater and detector strips and electrode pads havebeen already defined. It is to be noted that a combination of thesubstrate 51a and metal film 51b is also referred to as a "substrate 51"hereinbelow.

As shown in FIG. 28c, on the substrate 51 is formed a Sn film 52 to thethickness preferably ranging from 0.5 to 3 microns, for example, byevaporation or sputtering. In this instance, if a hydrate of Sn iscreated in the Sn film 52, the resulting SnO₂ film will be too sensitiveto humidity so that there will be a lack of stability and reliabilitywhen used as an element of a gas detecting device as described above.Furthermore, difficulty will be encountered in converting into a SnO₂film in the later described step of producing a thin film of oxide bythermal decomposition. Accordingly, it is important that no water iscontained in or absorbed into the Sn film 52 during its formation.Moreover, since the surface of Sn film 52 is active, it is preferable tomake the film 52 dense, and as small in surface area as possible. Forexample, with the atmosphere inside a vacuum chamber maintained at1×10⁻⁶ Torr or less and after removing the residual gas and absorbinggas, in particular, H₂ O by bake-out or a trap of liquid nitrogensufficiently, a thin film of Sn is formed by evaporation with theapplication of heat or sputtering in an Ar atmosphere at pressureranging from 1×10⁻⁴ to 1×10⁻³ Torr. The film forming rate or depositionrate is preferably kept at a relatively slow rate ranging from 0.01 to0.1 microns/min, thereby allowing to obtain a dense thin film 52 of Sn.

FIG. 28b schematically shows the case in which the Sn film 52 is formedby the resistive heating evaporation method. In this case, anevaporation heater 54 heats tin pellets 54' to be evaporated anddeposited onto the substrate 51.

Then, as shown in FIG. 28d, on the Sn film 52 is formed a photoresist 53which is selectively removed by photolithography thereby forming adesired pattern. Then, using the thus pattered photoresist 53 as a mask,the Sn film 52 is dipped into an aqueous solution of nitric acid of 2%by volume or more (room temperature) so that the exposed portions of Snfilm 52 are converted into white, cotten-like deposits in 10-60 seconds,which are then removed by water washing or ultrasonic cleaning (FIG.28e). Then the remaining photoresist 53 is removed to provide the Snfilm 52 having a desired pattern, as shown in FIG. 28f.

Then the Sn film 52 thus obtained is dipped into an aqueous solution ofnitric acid of 0.1-5% by volume (5°-25° C.), which causes to producewhite, cotton-like Sn and a film 52' of dilute nitric acid reactant in0.5-10 minutes (FIG. 28g). These Sn and dilute nitric acid reactant film52' are then heated to have them thermally decomposed thereby convertingthe film 52' into an oxide film 52" of SnO₂ (FIG. 28h). The heating maybe carried out using an electric furnace which is heated to thetemperature ranging from 400° to 600° C. approximately for 1-10 minutesunder the atmospheric condition. In this event, if the Sn and diluenitric acid reactant film 52' on the substrate 51 is observed underillumination by a light source emitting white light, the color changesin the sequence of white-yellow-brown-red-black-white or colorless andtransparent as the temperature of the furnace increases. This indicatesthe sequence of producing the SnO₂ film 52" without containing a hydrateof Sn. The resulting SnO₂ film 52" may be used as a gas detectingelement as described previously.

In the above-described process, since patterning may be carried out byphotoetching, a gas detecting film having an extremely fine patternwhich has not been obtained in the prior art may be obtained with easeand under control. Besides, in the present process, the acid employed islow in concentration, the reaction temperature is low and the processingtime is short, so that no corrosion occurs to the material forming thesubstrate 51, and, thus, the required film may be produced withoutdegrading reliability in operation.

Another process for manufacturing a film of metal oxide semiconductorwhich is particularly suited for use as a gas detecting film will now bedescribed with reference to FIGS. 29a through 29h. This embodiment isdirected to the formation of a gas detecting film on a bridge-formedsupporting structure. As described before, such a bridge-formedsupporting structure is particularly advantageous because a void space51c is formed under the supporting layer 51b on which the gas detectingfilm is to be formed so that the structure can provide a high thermalresponse and uniform temperature distribution.

FIG. 29a shows a starting structure in the present process, whichincludes the substrate 51a and the metal film 51b formed on thesubstrate 51a. Similarly with the previous embodiment, the metal film51b is suitably patterned to define heater and detector strips andelectrode pads. In this case, however, the substrate 51a is formed withthe void space 51c adjacent to the underside of at least part of themetal layer 51b thereby providing a bridge-formed supporting structure.In the present embodiment also, a combination of the substrate 51a andmetal layer 51b will be called "substrate 51" hereinbelow.

As shown in FIG. 29b, a mask 55 is placed above the substrate 51. Themask 51 has an opening which exposes only a predetermined region of thebridge section and covers the remaining surface of substrate 51 entirelywhen set in position. Using this mask 55, and Al film 57 having adesired pattern is formed on the substrate 51, a shown in FIG. 29d. FIG.29c shows the case in which the Al film 57 is formed using the resistiveheating evaporation method such that Al pellets 56' are heated by anevaporation heater 56 to be evaporated and deposited onto apredetermined region of the bridge section of substrate 51 through theopening of mask 55. The Al film 57, on the other hand, may also beformed by other evaporation methods or sputtering under the conditionswhich have beed described with respect to the formation of the Sn film52 in the previous embodiment.

Then, using the mask 55 again, Pt sputtering (Ar pressure 1×10⁻¹ -10Torr) is carried out to form a porous film 58 of Pt on the Al film 57,as shown in FIG. 29e, to the thickness of 0.02 to 0.06 microns. And,then, using the mask 55 again, Pd sputtering (Ar pressure 1×10⁻¹ -10Torr) is carried out to form a porous film 59 of Pd on the Pt film 58 tothe thickness of 0.02 to 0.06 microns, as shown in FIG. 29f.

Then, the thus formed Al film 57, Pt film 58 and Pd film 59 are dippedin an aqueous solution of nitric acid of 0.2 to 2% by volume at 5°-25°C., whereby the aqueous solution becomes absorbed into the Pt film 58and the Pd film 59 through their porous surfaces thereby producing adilute nitric acid reactant film 57' due to a reaction between the Alfilm 57 and the impregnated dilute nitric acid, as shown in FIG. 29g.The film 57' formed by a reactant between Al and dilute nitric acid isthen heated to have it thermally decomposed thereby producing an oxidefilm 57" of Al₂ O₃. After this thermal decomposition, Pt 58' and Pd 59'are present at the surface of Al₂ O₃ film 57" as distributed indispersion, as shown in FIG. 29h. It is to be noted that the conditionsfor thermally decomposing the film 57' of a reactant produced from areaction between Al and dilute nitric acid are the same as describedwith respect to the formation of the film 52' from a reactant producedfrom a reaction between Sn and dilute nitric acid.

The gas detecting semiconductor film thus produced may be generallycategorized in the so-called contact combustion type catalyst. However,as differnt from the prior art catalyst, since the present film isproduced by thin film forming technology, an extremely fine pattern maybe obtained. Moreover, the resulting film is stable as a catalyst for anextended period of time because the Al₂ O₃ film 57" obtained from thethermal decomposition of the Al-dilute nitric acid reactant film 57' isthermally quite stable and capable of holding Pt 58' and Pd 59' inposition strongly.

It is to be noted that the present film forming method may beadvantageously applied to the formation of a transparent electrode filmwhich is often required in a device such as a liquid crystal displaypanel and a plasma display panel. Thus, as a further embodiment of thepresent invention, there will be described a process for forming atransparent electrode film of tin oxide on the surface of a glasssubstrate with reference to FIGS. 30a through 30g.

At the outset, as shown in FIG. 30a, the thin film 52 of Sn is formed onthe substrate 51 which is glass in the present embodiment. Then, asshown in FIG. 30b, a film 60 of Au is selectively formed in apredetermined region on the Sn film 52 using a mask in the well knownthin film forming technology. Then, as shown in FIG. 30c, thephotoresist 53 is formed covering the Au film 60 and Sn film 52, whichis then patterned using the well known photolithographic technology.Then, as shown in FIG. 30d, undesired portions of Sn film 52 areremoved, which is followed by the step of removing the remainingphotoresist 53 entirely from the structure using a resist separatingagent. Then the remaining Sn film 52 is dipped into an aqueous solutionof nitric acid thereby producing the film 52' which is formed by areactant from a reaction between Sn and dilute nitric acid, as shown inFIG. 30e. Then with the application of heat, the film 52' is thermallydecomposed to produce the film 52" and SnO₂, as shown in FIG. 30f. Inthis instance, since the underlying portion below the Au film 60 doesnot come into contact with the aqueous solution of nitric acid, itremains as Sn but it reacts with the Au film 60 during the step ofthermal decomposition, thereby forming an eutectic alloy 61 of Au - Snsystem conveniently. such an Au - Sn system eutectic alloy 61 may beused as a bump for thermocompression bonding, so that a lead 62 may bethermocompression-bonded to the bump 61, as shown in FIG. 30g.

It is to be noted that the films 52, 52' and 52" in the presentembodiment may be formed as in the manner described with respect to theprevious embodiment of forming a gas detecting semiconductor film on aheat-resistant substrate. It is to be noted that in accordance with thepresent invention a transparent electrode film is formed by photoetchingso that a very fine pattern may be defined with ease and bonding may becarried out securely as well as easily. For example, in the prior artliquid crystal display panel, a connection to its transparent electrodefilm from a driving circuit is made by an electrically conductive rubbercontact. On the other hand, in accordance with this aspect of thepresent invention, such a connection may be made by wire bonding therebyallowing to increase reliability in operation.

FIG. 31 shows the overall structure of a minute sized gas detector 100employing a micro-heater. FIG. 32 is a cross-sectional view taken alongline I'--I' indicated in FIG. 31. As shown, on a substrate 101 is formedan insulating layer 102 on which is also formed a metal layer 103, whichis patterned to define three separate strips 103a, 103b and 103c eachprovided with electrode sections (A-F) on both ends. The substrate 100is provided with a rectangularly shaped recess 101a located generallycentrally at its top surface, thereby defining a bridge-like structure.Furthermore, a gas detecting layer 104 is formed partly covering thethree strips 103a-103c as shown. Among the three strips, the side strips103a and 103c are heater strips, which produce head due to Joule heatingwhen an electric current is passed therethrough, and the center strip103b is a detector strip for producing a detection signal indicating thepresence or overpresence of a particular gas to be detected.

As shown in FIG. 33, the bridge section extending above the void space101a formed in the substrate 101 has dimensions such that 1=40 microns,m=500 microns and the area of bridge section=2×10⁴ microns squared. FIG.34 shows a driving circuit which may be used to drive the gas detector100. When a voltage pulse is applied from a source P between theelectrodes A and D and C and F, the gas detecting layer 104 becomesheated by the heater strips 103a and 103c so that the value of itselectrical resistance lowers. When the gas detecting layer 104 absorbs agas, the value of its resistance drops by 2 to 3 orders of magnitude,and, as a result, the current passing through the heater strips 103a and103c is partly by-passed into the center detector strip 103b. Therefore,the concentration of gas may be detected by observing the changes involtage between the electrodes B and E. In the circuit of FIG. 34, ifthe resistances between electrodes A and C and D and F are both suchthat R₀ =56 ohms at room temperature, then it will be R=77 ohms (R/R₀=1.38) for I_(P) =22 mA and V_(P) =1.7V, so that approximately 37 mW ofpower will be consumed.

FIG. 35 is a cross-sectional view taken along line II'-II' indicated inFIG. 31 and shown on an enlarged scale schematically. In FIG. 35, heatconduction from the heater strips 103a and 103c is indicated by thearrows, and it will be appreciated that the temperature of heated gasdetecting layer 104 will not be uniform especially in the traversedirection. Disadvantages such as inefficient heating, a reduction in gasabsorption and difficulty in preferential detection of a particular gasmay be brought about. Moreover, as shown in FIG. 36, in which thedirections of heat conduction in the side heater strips 103a and 103care indicated by the arrows, the heat produced at the central portionsof the heater strips 103a and 103c partly escape to the electrodesections A and C, thereby causing power loss in heating. This isbecause, each of the electrode sections A and C is normally formed tohave a surface area which is significantly larger than the surface areaof the bridge section and in contact with the substrate 101 whichfunctions as a heat sink. Furthrmore, the heater strips 103a and 103care formed from a metal material having a relatively larger thermalconductivity.

Now, a gas detector constructed in accordance with a further embodimentof the present invention which is free of the above-described problemswill be described below. FIG. 37 shows in plan view an improved gasdetector 120 and FIG. 38 is a cross-sectional view taken along lineIII'--III' indicated in FIG. 37. This gas detector 120 includes asubstrate 121 provided with a generally rectangularly shaped void space125 and an insulating layer 122 having a pair of base sections formed onthe corresponding ridges of substrate 121 and an elongated sectionextending between the base sections thereby forming a bridge-shapedstructure. On each of the base sections is formed an electrode sectionor pad 123a or 123b. On the elongated section defining the bridge-shapedstructure is formed a gas detecting layer 124 as extending between theelectrode sections 123a and 123b. The substrate 121 is formed from amaterial which is heat-resistant and which may be easily undercut duringetching without causing any damages to the overlying structure.

The preferred materials for substrate 121 include Si, Cr, Ni, Mo, NiCrand stainless steel. The substrate 121 preferably has the thicknessranging from 0.1 to 1 mm. The insulating layer 122 is formed from amaterial which is highly heat-resistant and electrically insulating,such as Si₃ N₄, SiO₂, SnO₂, TiO₂, Ta₂ O₅, MgO, Al₂ O₃ and ZrO₂, to thethickness of 0.5-5 microns. The electrode sections 123a and 123b areformed from a material having a high electrical conductivity, such asTi, Ni, Cr, NiCr, Au, Pt, Rh, W, Mo, a metal carbide like WC, a metalsilicide like PtSi and a metal nitride like Ta₂ N, to the thickness of0.5-5 microns. The gas detecting layer 124 is formed from a metal oxidesemiconductor material, such as SnO₂, Fe₂ O₃ and ZnO, to the thicknessof 0.5-5 microns.

It will now be described as to the principle of operation of the gasdetecting device 120 shown in FIG. 37. It is to be noted that the metaloxide semiconductor forming the gas detecting layer 124 is in factcomprised of a collection of fine particles as schematically indicatedin FIG. 39. Observed microscopically, contact points or area between theadjacent particles are small so that the contact resistance isrelatively large. As a result, when current is passed through this layer124, more Joule heating takes place at the contact points between theparticles rather than at the bulk of each of the particles. Thetemperature distribution along the current path indicated in FIG. 39 isgraphically shown in FIG. 40, in which the abscissa is taken for thecurrent path and the ordinate is taken for temperature. The locationsindicated by A, B and C on the abscissa correspond to the locations A, Band C, respectively, shown in FIG. 39. It is shown in FIG. 40 that thetemperature is higher at contact points between particles than at thebulk of each particle. As described before, the gas detecting layer 124is formed on the bridge-shaped insulating layer 122 which extends abovethe void space 125 provided at the top surface of the substrate 121,and, moreover, it is extremely small in thermal capacity due to itsminuteness in structure, so that it is easily self-heated to a desiredtemperature level sufficient for absorption of gas.

FIG. 41 shows an example of a driving circuit which may beadvantageously applied to drive the gas detecting device 120 of FIG. 37.As shown, the gas detector 120 is connected as a detecting element inthe form of a well known bridge circuit. The driving circuit alsoincludes a temperature compensating element 119 which is, in fact,comprised of the same gas detector 120 as completely enclosed, aresistor 111 and a variable resistor 112. The driving circuit receivespower from a source such as a battery or a pulse generator, and changesin output voltage V_(OUT) between terminals 114 and 115 are monitored todetect presence or overpresence of gas. Since the gas detector 120 issusceptible to changes in the sorrounding atmospheric temperature, it ispreferable to be driven by a bridge-formed driving circuit as shown inFIG. 41.

With the area of the gas detecting layer 124 shown in FIG. 37 equal tothe total area of the heater strips on the bridge section shown in FIG.33, if the driving voltage for the gas detector 120 is 1.7V, then thevalue of current is 0.5 mA and the power consumption is 0.85 mW. Inaccordance with the present invention, since the size may be reducedmore without problem, for example a gas detector whose area of gasdetecting layer is in the order of 1 micron squared, a further reductionof power consumption is possible as the device is made smaller. Besides,under the condition, if the gas detector 120 is used as a gas leak alarmin combination with the driving circuit of FIG. 41, an increase of 10 mVin output voltage for the presence of 0.4% of isobutane in theatmosphere as compared with the atmosphere having no isobutane, whichindicates sufficient ability of gas detection. In comparison, in the gasdetection 100 of FIG. 31, the driving current is 22 mA and the powerconsumption is 37 mW. As a result, the structure shown in FIG. 37 ismore advantageous and highly efficient because the driving current andpower consumption may be reduced to 1/4 and 1/40, respectively, ascompared with the structure of FIG. 31. This is believed to be based onthe phenomenon of the temperature being higher at the contact pointsbetween the adjacent particles where the effect of gas absorption ishigher in the metal oxide semiconductor film serving as a gas detectingfilm, and, thus, the input power may be reduced since it may be avoidedto heat the bulk of each of the particles which does not participate somuch in absorption of gas. In addition, the temperature of the bridgesection as a whole is not increased significantly and thus there will beless aging effects.

Now, a process for manufacturing the gas detector 120 will be describedwith reference to FIGS. 42-48. In the first place, on the substrate 121is formed the insulating layer 122 using the well known film formingtechnology such as evaporation, sputtering and CVD (FIG. 42). Then onthe insulating layer 122 is formed the electrode layer 123, for example,by evaporation or sputtering (FIG. 43). Thereafter, using the well knowphotolithography, the electrode layer 123 is selectively removed todefine the electrode sections 123a and 123b (FIG. 44). For example, ifthe electrode layer 123 is formed from Ti, then the layer 123 may beselectively etched by an aqueous solution of 20-50% HF (liquidtemperature 30°-35° C.) for 0.5-5 minutes using photoresist as a mask.Furthermore, the insulating layer 122 is patterned by photolithography(FIG. 45). The thus patterned insulating layer 122 has a pattern whichmay be used as a mask in forming the void space 125 by etching thesubstrate 121 and as a supporting bridge structure for supportingthereon the gas detecting layer 124. That is, if the substrate 121 is Si(100), then it is so selected that the end of the supporting bridgestructure is at 45° with respect to the Si (111) plane of the substrate121 in order that the void space 125 may be formed under the insulatinglayer 122 by applying the well known anisotropic etching. If theinsulating layer 122 is SiO₂, it may be etched by buffered fluoric acidliquid (liquid temperature 30°-40° C.) using photoresist as a mask for1-10 minutes.

Then, using the patterned insulating layer 122 as a mask, the substrate121 is subjected to anisotropic etching to form the void space 125 (FIG.46) to the depth ranging from 20 to 100 microns. For example, if thesubstrate 121 is Si, use may be made of such anisotropic etching liquidas ethylenediamine+catechol +water (liquid temperature 90°-120° C.) andan aqueous solution of 20-70% NaOH (liquid temperature 80°-130° C.).Then the gas detecting film 124 is formed on the insulating layer 122defining a supporting bridge structure. The film 124 is formed longenough to contact or partly overlap each of the electrode sections 123aand 123b on both ends (FIG. 47). The formation of film 124 may becarried out for example by deposition of a metal oxide semiconductormaterial by evaporation or sputtering through an opening defined in ametal mask. Finally, leads 126a and 126b are connected to the electrodesections 123a and 123b, respectively, to complete the gas detector (FIG.48).

FIG. 49 illustrates a gas detector 120' constructed by modifying theabove-described embodiment. FIG. 50 is a cross-sectional view takenalong line IV'--IV' indicated in FIG. 49. The gas detector 120' isstructurally similar to the gas detector 120 shown in FIGS. 37 and 38 inmany respects. In the present embodiment, however, the connectionbetween the gas detecting film 124' and each of the electrode sections123a and 123b is defined at a position on the supporting bridge section.As a result, in the gas detector 120', elongated leads 123a' and 123b'extend along the bridge section from the electrode sections 123a and123b, respectively. Such a structure contributes to enhance uniformityof the temperature distribution in the gas detecting film 124', whichthen increases selectivity in the kinds of gas to be detected at aparticular temperature.

A further modification of the above-described embodiment is illustratedin FIG. 51. As shown, in a gas detector 120", the bridge section is notstraight but it is made wider toward the center, where the temperaturetends to be higher along the lengthwise direction. With such astructure, the temperature distribution in the gas detecting film 124"may be made even more uniform. In addition, since the current densitymay be made smaller at the location where the temperature tends to behigher, no deterioration in performance will occur and thus service lifemay be extended.

FIGS. 52 through 55 show cross-sectional views at several steps inanother process for manufacturing a gas detector 130. As shown in FIG.52, an insulating layer 132 is first formed on a substrate 131. Then theinsulating layer 132 is suitably patterned and then using the thuspatterned insulating layer the substrate 131 is selectively etched toform a void space 135 thereby defining a bridge structure by thepatterned insulating layer 132. Then, using metal mask, a gas detectingfilm 134 is formed on the bridge-formed insulating layer 132 from ametal oxide semiconductor material by evaporation or sputtering. Then,using another metal mask, electrode sections 133a and 133b are formed.In the above-described process for manufacturing the gas detector 130,no etching is required to form the electrode sections 133a and 133b sothat the process is simplified. It is to be noted that as a modificationof the structure shown in FIG. 55, the bridge section may be formed as acantilever structure as illustrated in FIG. 56.

FIGS. 57 through 60 show cross-sectional views at several steps in afurther process for manufacturing a sill further embodiment, gasdetector 140, of the present invention. The gas detector 140 includes asubstrate 141 which is formed from an electrically conductive material.As shown in FIG. 57, an insulating layer 142 is formed at the bottom ofthe substrate 141. Then the substrate is selectively etched until theinsulating layer 142 is reached to define a void space 145 in thesubstrate 141. Thereafter, using a metal mask, a gas detecting film 144is formed on the insulating layer 142 within the void space 145 from ametal oxide semiconductor material. As an alternative, after drying,baking and grinding a neutralized sediment of tin chloride, theresulting powder is dispered in an organic solvent, which may be appliedto the interior of the void space 145 to form the gas detecting film144. In the case, since the substrate 141 is electrically conductive,there is no need to form separate electrode sections and leads 146a and146b may be directly connected to desired portions of the substrate 141.FIG. 60 is a plan view of the gas detector 140 shown in FIG. 59.

FIG. 61 illustrates a still further embodiment, gas detector 150, of thepresent invention. The gas detector 150 includes a substrate 151 whichis comprised of an electrically insulating material, such as glass,SiO₂, Al₂ O₃, and MgO, or a highly heat-resistant film asfluoroplastics, polyimid, epoxy resin, and silicon resin, and extremelythin in the order of 0.01-1 mm. On top of the substrate 151 is formedelectrodes 153a and 153b and a gas detecting film 154 with leads 156aand 156b connected to the electrode 153a and 153b, respectively. In thisembodiment, since the gas detector 150 employs the extremely thinsubstrate 151 which is electrically insulating and small in thermalcapacity, steps of photoetching of the insulating layer 122 andundercutting of the substrate 121 in the case of the gas detector 120shown in FIGS. 37 and 38 need not be carried out.

FIGS. 62 through 67 are cross-sectional views at several steps formanufacturing a even still further embodiment, gas detector 160, of thepresent invention. As shown in FIG. 62, at the outset, a gas detectinglayer 174 is formed on an electrically insulating substrate 161 ofceramics, highly heat-resistant resin, etc. The gas detecting layer 164may be formed, for example, from a metal oxide semiconductor material byevaporation, sputtering, CVD, or the like to a desired pattern to thethickness of 0.5-5 microns. As an alternative, after processing throughthe well known wet process, the layer 164 may be formed by screenprinting or spin coating. Then using a metal mask an insulating layer162 is formed by evaporation, sputtering or the like and then it isselectively etched to define a pair of openings. Then using a metal maskelectrodes 163a and 163b are formed as partly filled in the respectiveopenings and leads 166a and 166b are bonded to the electrodes 163a and163b, respectively. Furthermore, in the case where the substrate 161 hasa thickness which is larger by ten times or more than the gas detectinglayer 164, undercutting is carried out, as shown in FIG. 65 or 66, toform a void space 165 thereby allowing to reduce power consumption. FIG.67 is a plan view of the gas detector 160.

FIGS. 68 through 70 are cross-sectional views at several steps of astill further process for fabricating a still further embodiment, gasdetector 170, of the present invention. In the first place, a gasdetecting layer 174 is formed on an electrically insulating substrate171. For example, the gas detecting layer 174 is formed from a metaloxide semiconductor material to the thickness of 5-100 microns. Then thesubstrate 171 is subjected to undercut etching to form a void space 175as shown in FIGS. 69 or 70. On the other hand, on the gas detectinglayer 174 is formed a pair of electrodes 173a and 173b by evaporation,sputtering or the like using a metal mask, and then leads 176a and 176bare bonded to the electrodes 173a and 173b, respectively. It is to benoted that in the present embodiment the gas detecting layer 174 definesa bridge structure by itself. Thus, the gas detecting layer 174 in thiscase preferably has the thickness raging from 5 to 100 microns in orderto have a sufficient mechanical strength against externally appliedforces and vibrations. However, its upper limit in thickness should bedetermined by power consumption because the thicker the gas detectinglayer 174, the larger the power consumption. Alternatively, themechanical strength of gas detecting layer 174 may be increased byhaving a binder, such as silica and alumina, mixed with a metal oxidesemiconductor material when forming the gas detecting layer 174.

What is claimed is:
 1. A method for manufacturing a film of metal oxidecomprising the steps of:forming a thin film of metal on a substrate;reacting said thin film of metal with a dilute nitric acid therebyconverting the film into a first film of a reactant produced in areaction between said metal and said dilute nitric acid; and thermallydecomposing said first film thereby converting said first film into afilm of a metal oxide which is produced during the thermaldecomposition.
 2. A method of claim 1 wherein said metal comprises Sn.3. A method of claim 1 wherein said metal comprises Al.
 4. A method ofclaim 2 wherein said metal oxide film is a gas detecting film whichchanges its electrical resistance when it absorbs a gas.
 5. A method ofclaim 2 wherein said metal oxide film is a transparent electrode film.6. A method of claim 3 further comprising the step of depositing Pt andPd onto said film of metal prior to the step of reacting.